1. Field of the Invention
The present invention relates to an optical multiplexer/demultiplexer for multiplexing or demultiplexing light with different wavelengths.
2. Related Background Art
An optical multiplexer/demultiplexer combines light signals with different wavelengths into one multi-wavelength light signal, or separates one multi-wavelength light signal into light signals with different wavelengths. The optical multiplexer/demultiplexer is an indispensable optical component for a WDM (Wavelength Division Multiplexing) transmission system for transmitting multi-wavelength signals. In a WDM transmission system, light signals to be transmitted by one optical fiber transmission line are multiplexed by an optical multiplexer at the optical transmitter side, and the multiplexed light signal is demultiplexed by an optical demultiplexer at the optical receiver side.
For such an optical multiplexer/demultiplexer, an optical multiplexer/demultiplexer, which includes an AWG (Arrayed Waveguide Grating) or a reflection grating, can be used. While an AWG is expensive, a reflection grating is superb in mass production, and is relatively inexpensive. This is because many replicas can be easily created from one shape of a grating. Therefore, recently optical multiplexers/demultiplexers which include a reflection grating are being commercialized.
For example, the optical multiplexer/demultiplexer disclosed in Japanese Patent Laid-Open No. 7-77627 comprises a plurality of optical waveguides formed on a planar substrate, a reflection grating and a lens. In the optical multiplexer/demultiplexer, the lens is disposed between the end faces of the optical waveguides and the grating. If a multi-wavelength light signal enters one of the optical waveguides, the light signal is emitted from the end face of the optical waveguide. The light signal reaches the grating through the lens. The wavelength components of the light signal are diffracted by the grating at angles according to the wavelengths. These wavelength components enter the lens at different angles from one another, and individually enter the end faces of the other optical waveguides. Thus, the multi-wavelength light signal is demultiplexed into light signals with different wavelengths. In the reverse propagation path, light signals with different wavelengths are multiplexed into a multi-wavelength light signal.
In the optical multiplexer/demultiplexer disclosed in the above publication, light signals with different wavelengths diffracted by grating travel in the different directions. Therefore, in order to efficiently send light signals with different wavelengths into the end faces of the corresponding waveguides, sophisticated lens design and adjustment of the optical system are required.
It is an object of the present invention to provide an optical multiplexer/demultiplexer which can multiplex/demultiplex light efficiently and which can be manufactured at low cost.
An optical multiplexer/demultiplexer according to the present invention comprises a first port, one or more second ports, and first and second diffraction gratings. The first grating receives and diffracts light from the first port. The second grating diffracts the light diffracted by the first grating to direct the light to the one or more second ports. The second grating is disposed parallel with the first grating. The second grating may have a diffraction surface parallel with a diffraction surface of the first grating. The second grating has the same grating interval and grating direction as the first grating. The multiplexer/demultiplexer demultiplexes a multi-wavelength light signal incident on the first port into light signals with different wavelengths, and outputs at least one of the light signals through the one or more second ports.
The multi-wavelength light signal incident on the first port is diffracted by the first grating at diffraction angles according to the wavelengths to form the light signals with different wavelengths. The light signals are diffracted again by the second grating at diffraction angles according to the wavelengths. The first and second gratings are disposed parallel with each other, and both have the same grating interval and grating direction. Therefore, the light signals with different wavelengths diffracted by the second grating travel parallel with each other. Thus, the optical multiplexer/demultiplexer can send the light signals easily and efficiently to the corresponding second ports without sophisticated lens design and adjustment of the optical system. Furthermore, in the optical multiplexer/demultiplexer, the grating, whose replicas can be made at low cost, is used rather than an expensive AWG. Consequently, the optical multiplexer/demultiplexer can be manufactured at low cost.
The optical multiplexer/demultiplexer may comprise a plurality of second ports. In this case, the optical multiplexer/demultiplexer may multiplex light signals with different wavelengths incident on the second ports into a multi-wavelength light signal, and output the multi-wavelength light signal through the first port.
A mirror parallel with the first grating may be disposed on an optical path between the first and second gratings. In this case, the light incident on the first port is diffracted by the first grating, reflected by the mirror, and then diffracted again by the second grating. The first and second gratings may be integrated together. In this case, the optical system can be adjusted more easily. It is preferable that the reflectance of the mirror is 90% or more at a working wavelength band. In this case, loss in the transmission band may be small.
The optical multiplexer/demultiplexer may further comprise a first lens and one or more second lenses. The first lens may be disposed on an optical path between the first port and first grating. The first lens has an optical axis forming an angle xcex80 with the perpendicular of a diffraction surface of the first grating. The one or more second lenses may be disposed on optical paths between the second grating and the one or more second ports. The one or more second lenses have optical axes forming the angle xcex80 with the perpendicular of a diffraction surface of the second grating. In this case, multiplexing and demultiplexing are performed very efficiently.
It is preferable that f1xc2x7NA1 less than f2xc2x7NA2 is satisfied where a focal distance of the first lens is f1, a numerical aperture of a first optical waveguide to be coupled with the first lens is NA1, a focal distance of each second lens is f2, and a numerical aperture of each of one or more second optical waveguides to be coupled with the one or more second lenses is NA2. In this case, the transmission wavelength spectrum of light to be transmitted becomes wide and flat.
A slit device may be disposed between the one or more second lenses and second grating. The slit device includes one or more slits arranged on optical axes of the one or more second lenses. When the width of each slit along a direction perpendicular to both the optical axes of the one or more second lenses and grating direction of the second grating is S, the formula S less than 2xc2x7f2xc2x7NA2 is satisfied. In this case, the transmission wavelength spectrum of light to be transmitted becomes wide and flat. It is preferable that the slit width S is variable. In this case, a multiplexing/demultiplexing characteristic of the optical multiplexer/demultiplexer can be adjusted by modulating the slit width S.
It is preferable that an optical path length between the first and second gratings is variable. In this case, the multiplexing/demultiplexing characteristic of the optical multiplexer/demultiplexer can be adjusted by modulating the optical path length.
The optical multiplexer/demultiplexer may further comprise a polarization separating element, polarization plane paralleling means, polarization plane orthogonalizing means, and polarization combining element. The polarization separating element polarizes and separates the light from the first port into a first light polarized in a first direction and a second light polarized in a second direction. The first direction is parallel with the grating direction of the first grating. The second direction is perpendicular to the first direction. The polarization plane paralleling means receives the first and second light from the polarization separating element. The polarization plane paralleling means rotates at least one of polarization planes of the first and second light to match polarization directions thereof, and directs the first and second light to the first grating. The polarization plane orthogonalizing means receives the first and second light from the second grating. The polarization plane orthogonalizing means rotates at least one of the polarization planes of the first and second light to orthogonalize the polarization directions thereof. The polarization combining element receives the first and second light from the polarization plane orthogonalizing means to polarize and combine the first and second light, and sends the combined light to the one or more second ports. In this case, the polarization status of the light incident on the first and second gratings is constant regardless of the polarization status of the light incident on the first port. Therefore, a stable transmission characteristic can be obtained.
The polarization plane paralleling means may rotate the polarization plane of the second light by 90xc2x0 so that the second light becomes polarized in the first direction. In this case, the first and second gratings can diffract incident light at high efficiency, so that the optical loss of the optical multiplexer/demultiplexer becomes small.
It is preferable that the optical path of the first light and the optical path of the second light are shifted in the grating direction of the first grating between the polarization plane paralleling means and the polarization plane orthogonalizing means. In this case, restriction in arranging the polarization plane orthogonalizing means and the polarization plane combining element is relaxed.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings 0which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.